Two events, A and B, have space time coordinates $(x_1, y_1, z_1, t_1)$ and $(x_2, y_2, z_2, t_2)$ in the frame S and coordinates $(x'_1, y'_1, z'_1, t'_1)$ and $(x'_2, y'_2, z'_2, t'_2)$ in the frame S'. S moves at a constant speed $v$ relative to S'. Suppose that A and B are simultaneous in S'. The fact that "simultaneity is relative" means
Select one:
a. $t_1 \neq t_2$
b. $t'_1 \neq t'_2$
c. $t'_1 \neq t_1$
d. $t'_2 \neq t_2$